1. Field of the Invention
This invention relates to a process for producing a polyoxymethylene-polyurethane type alloy, more particularly to a process for producing a poly-oxymethylene-polyurethane type alloy which comprises allowing a polyisocyanate compound to react with polyols in the presence of a polyoxymethylene compound.
The composition of the present invention is useful as construction materials for industry and various functional material parts, etc.
2. Related Background Art
Polyoxymethylene polymer and polyoxymethylene copolymer (hereinafter called comprehensively as POM) have well balanced mechanical properties, creep resistance as well as high elastic recovery, fatigue resistance, frictional resistance, chemical resistance, hot water resistance, etc., and also good workability, and hence called also as the engineering plastic of engineering plastics and have been used steadily and progressively in such fields of high degree of assembly working such as electrical, electronics industries or automobile industries and precise mechanical industries. However, they have the drawback in inferior flexibility and impact resistance.
On the other hand, polyurethane (hereinafter called PU) has been used in various fields, because it is excellent in abrasion resistance, low temperature characteristic, flexibility, impact resistance, etc. and its physical property can be easily varied over a wide range from soft to hard according to a combination of the constituent starting materials.
It has been proposed in the technique to make composites of POM and PU, which have been provided for uses such as gears for highly impact resistance, soft gears for prevention of noise, gears for ascending and descending windows of automobiles, and ski binding.
According to the composite formation methods of the prior art, all are restricted in aspect of workability and the scope of application, and had the drawbacks such that many complicated steps are required in the preparation of the alloy, that the molded products had tackiness, or that the strength at the welded portion of the molded product is extremely lowered.
More specifically, the following methods have been known for making composites of POM and PU.
(1) Blend of POM and a thermoplastic polyurethane:
In order to obtain a highly impact resistant and soft material within the range which does not lose the good essential properties of POM even if the elastic coefficient may be more or less lowered, it has been proposed to blend POM with a thermoplastic polyurethane (hereinafter called TPU) and practicably utilized in a part of the fields. However, since TPU is generally shaped in flakes or pellets of several mm, when blended with pellets or powder of POM, its processability is bad. Also, conventional TPU is poorly compatible with POM, particularly TPU with a hardness of 90.degree. or higher according to Type A of JIS A K6301 is substantially hardly compatible to give a non-homogeneous blend, thus involving the problems of insufficient mechanical strength of the molded product and bad appearance.
If blending is effected with elongated kneading time, elevated kneading temperature and strengthened shear force in order to increase homogeneity, formaldehyde through decomposition of POM will be vigorously generated to lower excessively the working environment.
Also, even if a homogeneous POM-PU composite may be obtained by use of a soft TPU, depending on the conditions during injection molding, interlaminer peeling phenomenon is liable to occur, whereby the strength of the molded product tends to be lowered.
(2) Modified oxymethylene polymer having POM and TPU coupled by polyisocyanate:
U.S. Pat. No. 3,364,157 discloses modified oxymethylene polymers which comprises oxymethylene polymer coupled with a dissimilar organic polymer having a terminal or pendant group containing active or acidic hydrogen as determined by the Zerewitnoff method by a polyisocyanate or polythioisocyanate.
Here, TPU is used as the dissimilar organic polymer, but the oxymethylene copolymer obtained according to such method is lowered in compatibility when the amount of TPU is much or depending on the composition to cause readily appearance badness of the molded product, although impact resistance can be improved.
Further, the molded product or extruded product prepared from the modified oxymethylene copolymer prepared by this method has the drawback that it is tacky.
(3) Preparation of modified oxymethylene copolymer in the presence of formaldehyde:
U.S. Pat. No. 3,766,298 discloses that the above drawback of tackiness can be improved when a modified oxymethylene copolymer is produced by the method as described in (2) in the presence of formaldehyde in an amount of about 10% by weight or less based on the total weight of the reaction components.
In this case, also similarly in the case of (1), formaldehyde will be generated greatly to worsen the working environment. Also, since TPU once synthesized is used, the production cost through all the steps will be disadvantageously high.
POM is a crystalline polymer, and excellent in workability due to its low melt viscosity but inferior in impact resistance. PU is a thermoplastic resin which gives a form through pseudo-crosslinking by hydrogen bond, having high melt viscosity with poor moldability, and hence has been desired to be improved in molding characteristics, and also POM in this case is desired to be improved in impact resistance. For accomplishing these objects, composite formation of POM-PU has been proposed, but since the essential properties as the resin are different as described above, with inherent poor compatibility between POM and PU and different behaviors during melting. Hence, even if they may be formulated uniformly forcibly by mechanical microdispersion, interlaminer separation is liable to occur in the place where great shearing force such as injection molding, etc. is received, which may be considered to cause surface peeling or lowering in physical properties.
Even when POM and TPU may be coupled according to the method as disclosed in U.S. Pat. Nos. 3,364,157 and 3,766,298, POM segments and TPU segments can be coupled completely with difficulty, whereby similar phenomenon may be considered to occur.
For solving these problems, it is necessary to bond POM molecular chains and PU molecular chains according to the chemical or physical method and also select a PU composition having good compatibility with POM.
The present inventors, in view of the state of the technique as described above, have studied intensively about a method for obtaining a composite resin with PU having good flexibility and impact resistance without losing the good properties of POM at low cost, and consequently found a method for obtaining an alloy of POM and PU having excellent physical properties and improved in the drawbacks of workability of the POM-PU composite of the prior art more simply than the method of the prior art to accomplish the present invention.